The securing of communications, especially mobile radio telephone communications, against casual or international eavesdropping suggests that the communications be encrypted. Since even encrypted communications can eventually be deciphered, a greater degree of security is obtainable if the key used for encryption is changed frequently.
Distributing new keys to a great number of mobile radio telephone units on a frequent basis has heretofore seemed to be an intractable problem. See, for example, Diffie and Hellerman, "Privacy and Authentication: An Introduction to Cryptography," Proceedings of the IEEE Vol. 67, No. 3, March 1979, page 400. It would of course be undesirable to transmit the new keys over the air. Moreover, in a high capacity "cellular" mobile radio telephone system such as that described in the January 1979 issue of The Bell System Technical Journal the new keys would also have to be distributed to the base stations in each of the cells located throughout the geographic service areas.
In a cellular radio telephone system the band of radio frequencies allocated by the federal authority is divided into different channels. A particular group of channels is assigned to each cell. This group of channels differs from the channels assigned for use by adjoining cells so that communications taking place in adjoining cells will not interfere with each other and more importantly, so that the channels in use in one cell can also be simultaneously put to use in more distant cells.
As a mobile unit leaves one cell and enters another, the new channel used by the mobile unit for communication with the base station will automatically be changed. Both the mobile unit and the base station in the new service area can be arranged to retain information concerning the identity of the channel frequency currently in use and that previously used.